Skip to content
Know.itKnow.it
Create an animated lesson
On request

This article is free to read. Creating a lesson uses your daily or stored credits; review the language and settings before starting.

How does a catalytic converter work?

TechnologyLesson on requestFree to read

Quick answer

A catalytic converter is a metal box in the exhaust holding a honeycomb coated with platinum, palladium and rhodium. As hot exhaust flows through, these metals help harmful gases react: carbon monoxide and unburned fuel become carbon dioxide and water, and nitrogen oxides become nitrogen. The metals themselves are not consumed.

A catalytic converter cleans car exhaust by passing it over a honeycomb coated with precious metals that speed up chemical reactions without being used up. Toxic carbon monoxide, unburned fuel and nitrogen oxides are turned into carbon dioxide, water vapour and nitrogen before the gas leaves the tailpipe.

Step by step

  1. Hot exhaust leaves the engine and enters the converter, warming the honeycomb to its working temperature.
  2. The gas spreads through thousands of narrow channels and touches the metal-coated walls.
  3. On rhodium sites, nitrogen oxides give up their oxygen and leave as nitrogen gas.
  4. On platinum and palladium sites, carbon monoxide and hydrocarbons combine with oxygen to form carbon dioxide and water.
  5. Oxygen sensors before and after the converter report to the engine computer, which adjusts fuelling to keep reactions balanced.

Key things to understand

  1. Exhaust gas flows through a ceramic or metal honeycomb with thousands of thin channels, giving a huge surface area.
  2. The channel walls carry a porous coating dotted with tiny particles of platinum, palladium and rhodium, which act as catalysts.
  3. Rhodium helps strip oxygen from nitrogen oxides, turning them into harmless nitrogen gas.
  4. Platinum and palladium help carbon monoxide and unburned hydrocarbons react with oxygen to form carbon dioxide and water.
  5. An oxygen sensor lets the engine computer hold the air-fuel mix near the chemically ideal ratio so all three reactions work.

Labelled diagram

  1. Exhaust inlet

    Carries carbon monoxide, hydrocarbons and nitrogen oxides from the engine.

  2. Honeycomb substrate

    Thousands of thin channels give the gas a huge surface to touch.

  3. Reduction catalyst

    Rhodium splits nitrogen oxides into nitrogen and oxygen.

  4. Oxidation catalyst

    Platinum and palladium turn carbon monoxide and hydrocarbons into carbon dioxide and water.

  5. Oxygen sensors

    Report exhaust oxygen so the engine computer can keep the mixture balanced.

Inside a three-way catalytic converter

Real-world examples

  • India's move to Bharat Stage VI emission rules in April 2020 required more advanced exhaust after-treatment on new vehicles.
  • A petrol car's check-engine light can come on when the rear oxygen sensor shows the converter is no longer working well.
  • Diesel trucks carry a separate tank of urea solution, often sold as AdBlue, for their selective catalytic reduction system.
  • Petrol motorcycles and scooters sold under modern emission rules also carry small catalytic converters in their exhausts.

Common mistake

✗ Myth: A catalytic converter removes carbon dioxide and makes exhaust climate-friendly.

✓ Fact: A catalytic converter actually produces carbon dioxide when it oxidises carbon monoxide and hydrocarbons. It targets toxic, smog-forming pollutants, not the greenhouse gas emitted by burning fuel.

People also ask

What does a catalytic converter do?
It reduces harmful pollutants in a vehicle's exhaust. Using precious metal catalysts on a honeycomb, it converts carbon monoxide into carbon dioxide, unburned hydrocarbons into carbon dioxide and water, and nitrogen oxides into nitrogen. This cuts smog and toxic gases released from the tailpipe.
What metals are in a catalytic converter?
The active metals are usually platinum, palladium and rhodium, spread as tiny particles over a porous coating on the honeycomb. Only a few grams are used, but their high value makes converters a target for theft. The outer case and substrate are steel and ceramic or metal foil.
Can a car run without a catalytic converter?
Mechanically the engine can still run, but the car will release far more pollution, usually trigger a warning light and fail emissions checks. Removing or disabling a catalytic converter is illegal in many countries, including on vehicles used on public roads.
Why does a catalytic converter need to get hot?
The catalyst only speeds up the reactions well once it reaches its light-off temperature, roughly 250–300 °C. Most of a car's tailpipe pollution is released in the first minutes after a cold start, before the converter has warmed up.
Why are catalytic converters stolen?
They contain small amounts of platinum, palladium and rhodium, which are valuable precious metals. Thieves can cut a converter from the exhaust pipe quickly, which is why some owners fit protective shields or marking.
How do diesel cars clean their exhaust?
Diesels run with extra oxygen, so a three-way converter cannot reduce their nitrogen oxides. They use an oxidation catalyst, a particulate filter to trap soot and often selective catalytic reduction, which injects a urea solution to turn nitrogen oxides into nitrogen and water.
What causes a catalytic converter to fail?
Common causes include engine misfires that send unburned fuel into the converter and overheat it, oil or coolant contaminating the coating and physical damage. Leaded petrol also poisons the catalyst, which is one reason lead was removed from fuel.

Part of the guide: Cars, Trains, Planes & Ships →